Raw natural gas contains primarily methane and also includes numerous minor constituents such as water, hydrogen sulfide, carbon dioxide, mercury, nitrogen, and light hydrocarbons typically having two to six carbon atoms. Some of these constituents, such as water, hydrogen sulfide, carbon dioxide, and mercury, are contaminants which are harmful to downstream steps such as natural gas processing or the production of liquefied natural gas (LNG), and these contaminants must be removed upstream of these processing steps. After these contaminants are removed, the hydrocarbons heavier than methane are condensed and recovered as natural gas liquids (NGL) and the remaining gas, which comprises primarily methane, nitrogen, and residual light hydrocarbons, is cooled and condensed to yield a final LNG product.
Because crude natural gas may contain 1–10 mole % nitrogen, removal of nitrogen is necessary in many LNG production scenarios. A nitrogen rejection unit (NRU) and/or one or more flash steps may be utilized to reject nitrogen from the LNG prior to final product storage. Nitrogen rejection requires additional refrigeration, and this refrigeration may be supplied by expansion of the feed to the nitrogen rejection system, by expansion of the recovered nitrogen-rich gas, by utilizing a portion of the refrigeration provided for liquefaction, or combinations thereof. Depending on the nitrogen rejection process, the rejected nitrogen still may contain a significant concentration of methane, and if so, this rejected nitrogen stream cannot be vented and must be sent to the plant fuel system.
In the production of LNG, liquefaction typically is carried out at elevated pressures in the range of 500 to 1000 psia, and the LNG from the liquefaction section therefore must be reduced in pressure or flashed prior to storage at near-atmospheric pressure. In this flash step, flash gas containing residual nitrogen and vaporized methane product is withdrawn for use as fuel. In order to minimize the generation flash gas, the liquefaction process typically includes a final subcooling step, which requires additional refrigeration.
In certain LNG operations, the generation of fuel gas streams in the final steps of the liquefaction process may be undesirable. This reduces available options for disposing of rejected nitrogen, since venting is possible only if the rejected nitrogen contains low concentrations of methane, for example, below about 5 mole %. Such low concentrations of methane in the reject nitrogen can be attained only by an efficient nitrogen rejection unit, and this requires sufficient refrigeration to effect the nitrogen-methane separation.
There is a need in the LNG field for improved nitrogen rejection processes which minimize methane rejection and which integrate efficiently with the LNG refrigeration system. The present invention, as described below and defined in the appended claims, addresses this need by providing process embodiments for removing nitrogen from LNG with minimum methane loss, wherein the process integrates LNG production and storage with efficient refrigeration for nitrogen rejection and final product cooling.